Metal powder sintering paste and method of producing the same, and method of producing conductive material

ABSTRACT

There is a problem that when a silver powder sintering paste that is substantially free from resin is used, an organic solvent used as a dispersion medium bleeds, which results in contamination and wire bonding defects. In order to solve the problem, provided is a metal powder sintering paste that contains, as a principal component, silver particles having an average particle diameter (a median diameter) of 0.3 μm to 5 μm and further contains an anionic surfactant but is substantially free from resin.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a 37 C.F.R. § 1.53(b) continuation of U.S. application Ser. No. 15/477,868 filed Apr. 3, 2017, which claims priority on Japanese Patent Application No. 2016-075314 filed Apr. 4, 2016. The entire contents of each application is hereby incorporated by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present disclosure relates to a metal powder sintering paste, a method of producing the same, and a method of producing a conductive material.

2. Description of Related Art

Conventionally, in a power semiconductor device or a light-emitting device that has a light emitting element as a light source, it has been known that a metal powder sintering paste in which metal particles are dispersed in a dispersion medium such as an organic solvent is used when, for example, a power semiconductor element is placed on a mounting member. A metal powder sintering paste is placed between a mounting member and, for example, a power semiconductor element, and heating them at around 200° C. allows the metal particles in the metal powder sintering paste to sinter to one another and thereby they are joined together.

Furthermore, as a method of joining a light emitting element and a mounting member together, a method has been known in which an adhesive containing a resin or a lead-free solder containing eutectic crystals used therein is placed between a light emitting element and a mounting member.

However, since the lead-free solder generally has a melting point of at least 300° C., the member might be damaged due to member deterioration caused by heating at a high temperature or stress caused by the difference in coefficient of thermal expansion between a substrate and the member at the time of cooling after they are joined. Therefore, in the method using a lead-free solder, the options of usable members can sometimes be limited.

In a metal powder sintering paste, when, for example, silver is used as the metal powder, although silver has a melting point of approximately 962° C., which becomes the theoretical heat resistance limit, joining is carried out by heating at around 200° C. Therefore, the metal powder sintering paste is more advantageous in terms of joining temperature and heat resistance of the light-emitting device to be obtained, as compared to the lead-free eutectic crystal solder containing eutectic crystals used therein.

Furthermore, in a metal powder sintering paste, when gold is not contained as the metal powder, it is less expensive as compared to a gold-tin eutectic crystal solder that is often used for mounting a light emitting element.

There are some metal powder sintering pastes that contain resins. However, from the perspectives that sintering requires a high temperature and a resin component or the volatile portion of the resin contaminates peripheral members, the better ones are those that are free from resin and contain a volatile organic solvent as a dispersion medium.

As compared to a method using an adhesive containing resin, a method using a metal powder sintering paste is excellent in terms of the heat resistance and heat dissipation of a light-emitting device obtained thereby since resin degradation does not occur.

A method of producing a metal member joined body using a material for joining a metal member as a metal powder sintering paste is known, wherein the material for joining a metal member is, for example, in the form of a paste composed of heat sinterable metal particles and a flux and becomes a porous sintered material having a melting point equivalent to that of said metal particles when being heated to be sintered (for example, JP5301385B). Specifically, this method is a method of producing a metal member joined body, wherein the method includes interposing a paste-like material composed of silver particles or copper particles (A) and a liquid material (B) between a plurality of metal members, heating them at a temperature between 200° C. and 400° C. to sinter the silver particles or copper particles (A) to one another to become a porous sintered material, and thereby joining the plurality of metal members to each other, the silver particles or copper particles (A) have an average particle diameter (a median diameter D50) of 0.2 μm to 10 μm and a melting point higher than 400° C. and have heat sinterability, and the liquid material (B) is composed of (b) an oxide film removal activator, namely, hydrochloride salt or bromate salt of amines, carboxylic acid, or organic halide, (c) a thixotropic agent, and (d) a solvent, or is composed of (b) an oxide film removal activator, namely, hydrochloride salt or bromate salt of amines, carboxylic acid, or organic halide, and (d) a solvent, and the porous sintered material has a melting point equivalent to that of the silver particles or copper particles (A) and a porosity at cross-section of 5 to 50% by area.

Furthermore, as a metal powder sintering paste, a metal particulate dispersion also has been known in which metal particulates with a dispersant adsorbing on the surfaces thereof are dispersed in a dispersion medium, said dispersant having at least one of a carboxyl group and a hydroxy group in its molecule, and said dispersion medium having a dielectric constant of at least 12.0 (JP2007-200775A).

Furthermore, as a metal powder sintering paste, a metal particulate dispersing liquid has been known in which metal particulates with the surfaces thereof being coated with fatty acid and aliphatic amine are dispersed in a hydrophobic solvent containing a fatty acid derivative added thereto, the fatty acid derivative being fatty acid methyl ester or fatty acid ethyl ester having 12 to 20 carbon atoms (JP5778494B).

A volatile organic solvent that is used for metal powder sintering pastes has a lower viscosity than that of an adhesive containing resin. Therefore, fine irregularities of the substrate-side electrode surface to be joined often cause an organic solvent component to wet/spread. The wetting/spreading of the organic solvent component is a bleeding phenomenon caused by a capillary phenomenon. This causes bleeding in an unexpected portion in the substrate, which causes the organic solvent to leak onto the substrate bottom surface during the process or results in wire bonding defects due to contamination.

SUMMARY OF THE INVENTION

Therefore, the present disclosure provides a metal powder sintering paste that prevents bleeding from occurring, and a method of producing the same, as well as a method of producing a conductive material using the metal powder sintering paste.

A metal powder sintering paste according to an embodiment of the present disclosure contains, as a principal component, silver particles having an average particle diameter (a median diameter) of 0.3 μm to 5 μm and further contains an anionic surfactant but is substantially free from resin.

A method of producing a metal powder sintering paste according to an embodiment of the present disclosure includes mixing an anionic surfactant with silver particles having an average particle diameter (a median diameter) of 0.3 μm to 5 μm, wherein substantially no resin is mixed together.

A method of producing a conductive material according to an embodiment of the present disclosure includes a step of calcining the metal powder sintering paste.

The metal powder sintering paste of the present disclosure can prevent bleeding from occurring during heating.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, there are described a metal powder sintering paste and a method of producing a metal powder sintering paste as well as a conductive material and a method of producing a conductive material of the present embodiments.

The metal powder sintering paste of the present disclosure contains, as a principal component, silver particles having an average particle diameter (a median diameter) of 0.3 μm to 5 μm and contains an anionic surfactant but is substantially free from resin.

Furthermore, the method of producing a metal powder sintering paste of the present disclosure includes mixing an anionic surfactant with silver particles having an average particle diameter (a median diameter) of 0.3 μm to 5 μm, wherein substantially no resin is mixed together.

In the metal powder sintering paste of the present disclosure, the anionic surfactant exhibits the resistance to bleed provided by its electric field with respect to an electrode such as silver or gold that generally has a negative surface potential. This improves the leakage onto the substrate bottom surface and the wire bonding defects caused by contamination, which makes it possible to produce reliable semiconductor devices, etc.

[Silver Particles]

In the metal particles to be used for the metal powder sintering paste, silver particles are the principal component. This means that the content of the silver particles contained in the metal particles is, for example, at least 70% by mass, preferably at least 80% by mass, and more preferably at least 90% by mass. The silver particles to be used herein may be one type of silver particles having the same average particle diameter (median diameter) or a mixture of at least two types of silver particles that are different in average particle diameter. The silver particles have an average particle diameter (a median diameter) of 0.3 μm to 5 μm, preferably 1.0 μm to 4 μm, and more preferably 1.5 μm to 3.5 μm. This can lower the electrical resistance value. Metal particles to be used other than the silver particles can be those having an average particle diameter (a median diameter) of 0.1 μm to 15 μm, but the average particle diameter thereof is preferably 0.3 μm to 10 μm, more preferably 0.3 μm to 5 μm.

With respect to the silver particles, the content of the particles having a particle diameter of less than 0.3 μm is preferably not more than 5% by mass, more preferably not more than 4% by mass.

With respect to the silver particles, the content of the particles having a particle diameter of not more than 0.5 μm is preferably not more than 15% by mass, more preferably not more than 10% by mass.

The average particle diameter (the median diameter) of the silver particles can be measured by a laser diffraction method. The average particle diameter (the median diameter) denotes the value where the accumulated frequency by volume is 50%, which is obtained from a particle size distribution. Hereinafter, the average particle diameter denotes the median diameter unless otherwise noted.

Furthermore, the silver particles have a specific surface area of 0.4 m²/g to 1.5 m²/g, preferably 0.6 m²/g to 0.9 m²/g, and more preferably 0.66 m²/g to 0.74 m²/g. This can increase the area where adjacent silver particles are joined to each other, and since the addition of the silver particles results in a small increase in viscosity, the paste can contain many silver particles. This prevents the occurrence of voids and allows a high joining strength to be obtained. The specific surface area of the metal particles that are the main raw materials of the metal powder sintering paste can be measured by the BET method.

The shape of each silver particle is not limited. However, examples thereof include a spherical shape, a flat shape, a flake shape, and a polyhedral shape, but the flake shape is preferred. This is because the flake shape increases the area of each silver particle where it is in contact with adjacent silver particles and decreases the electric resistance. The shapes of the metal particles each having an average particle diameter within a predetermined range are preferably uniform. When two or more types of metal particles that are different in average particle diameter are mixed, the shapes of the metal particles with the respective average particle diameters may be the same or different from each other. For example, when two types of metal particles, namely, those with an average particle diameter of 3 μm and those with an average particle diameter of 0.3 μm are mixed together, the metal particles with an average particle diameter of 0.3 μm may have a spherical shape while the metal particles with an average particle diameter of 3 μm may have a flat shape.

The metal powder sintering paste contains silver particles as its principal component. In the metal powder sintering paste, one type or more of metal particles other than silver particles can be used and examples thereof include gold, copper, platinum, palladium, rhodium, ruthenium, iridium, and osmium.

The content of the silver particles is preferably at least 70% by mass, more preferably at least 85% by mass, and further preferably at least 90% by mass, with respect to the paste. This is because when the content of the silver particles is within the predetermined ranges, the joining strength of the resultant conductive material increases.

[Surfactant]

The metal powder sintering paste contains an anionic surfactant. Due to the electric field generated by the anionic nature, the anionic surfactant exhibits the resistance to bleed provided by its electric field with respect to an electrode such as silver or gold that generally has a negative surface potential.

This improves the leakage onto the substrate bottom surface and the wire bonding defects caused by contamination, which makes it possible to produce reliable semiconductor devices.

The surfactant has preferably a high volatility. Specifically, when the temperature is raised from around room temperature at 2° C./min in thermogravimetric-differential thermal analysis (TG-DTA), those in which the residue obtained at 350° C. is not more than 20% by mass with respect to the initial mass are preferable, and those in which the residue obtained at 350° C. is not more than 5% by mass are more preferable. This is because when the residue is not more than 20% by mass, volatile residues do not disturb sintering during calcining, which increases the joining strength.

The anionic surfactant is preferably a carboxylic acid type containing a carboxyl group or a salt thereof, more preferably a carboxylic acid type represented by Formula (I) below. R¹O(CH₂CH(R²)O)_(n1)CH₂COOR³  (I) [In the formula, R¹ is a linear or branched alkyl group having at least 7 carbon atoms, R² is any one of —H, —CH₃, —CH₂CH₃, and —CH₂CH₂CH₃, R³ is —H or alkali metal, and n1 is in the range of 2 to 5.1][036]

Furthermore, the anionic surfactant is more preferably a carboxylic acid type represented by Formula (II) below. R¹¹—C(O)N(R¹²)(CH₂)_(n2)COOR¹³  (II) [In the formula, R¹¹ is a linear or branched alkyl group having at least 7 carbon atoms, R¹² is any one of —H, —CH₃, —CH₂CH₃, and —CH₂CH₂CH₃, R¹³ is —H, NH⁺(C₂H₄OH)₃, or alkali metal, and n2 is in the range of 1 to 5.]

Moreover, the anionic surfactant is further preferably a carboxylic acid type represented by Formula (III) below. R²¹—CH═CH—(CH₂)_(n3)COOR²²  (III) [In the formula, R²¹ is a linear or branched alkyl group having at least 7 carbon atoms, R²² is —H or alkali metal, and n3 is in the range of 1 to 10.1]

Furthermore, the anionic surfactant is further preferably a carboxylic acid type represented by Formula (IV) below. R³¹—COOR²²  (IV) [In the formula, R³¹ is a linear or branched alkyl group or alkoxy group having at least 7 carbon atoms, which has been optionally substituted with OH or COOR³³ (R³³ is alkali metal), and R³² is —H or alkali metal.]

Moreover, the anionic surfactant is preferably a sulfonic acid type containing a sulfo group or a salt thereof, more preferably a sulfonic acid type represented by Formula (V) below. R⁴¹—SO₃R⁴²  (V) [In the formula, R⁴¹ is a linear or branched alkyl group, aralkyl group or alkenyl group having at least 7 carbon atoms, which has been optionally substituted with OH or COOR⁴⁸ (R⁴³ is an alkyl group), or an aralkyl group, and R⁴² is —H or alkali metal.]

Furthermore, the anionic surfactant is preferably a carboxylic-sulfonic acid type containing a carboxyl group, or a salt thereof and a sulfo group, or both salts thereof, more preferably a carboxylic-sulfonic acid type represented by Formula (VI) below.

[In the formula, R⁵ is a linear or branched alkoxy group having at least 7 carbon atoms or R⁵⁴—CONH— (R⁵⁴ is a linear or branched alkyl group having at least 7 carbon atoms), R⁵² and R⁵³ are —H or alkali metal, n5 is in the range of 1 to 8, n6 is in the range of 0 to 1, and n7 is in the range of 0 to 1.]

Moreover, the anionic surfactant is preferably of a phosphate ester structure, or a salt thereof, namely, a phosphate ester type, more preferably a sulfate ester type represented by Formula (VII) below. R⁶¹—O—PO(OR⁶²)OR⁶³  (VII) [In the formula, R⁶¹ and R⁶² each are a linear or branched alkyl group, and R⁶³ is —H or alkali metal.]

The content of the surfactant is preferably up to 10% by mass with respect to the paste. Furthermore, the content of the surfactant is preferably not more than 2% by mass with respect to the paste, because this allows the surfactant to be completely volatilized by calcining.

The surfactant has preferably a contact angle of at least 10 degrees with respect to a gold electrode with a surface roughness Ra of 0.04 μm because this prevents the dispersion medium contained optionally from bleeding. The contact angle of the surfactant contained in the metal powder sintering paste can be measured with a contact angle meter.

The surfactant is preferably in a liquid state at 25° C., because this reduces the solid content in the paste to allow a larger amount of silver powder to be contained, which tends to prevent voids from occurring.

[Organic Solvent]

The metal powder sintering paste contains preferably an organic solvent as a dispersion medium. This is because uniform dispersion of the silver particles in an organic solvent allows a high quality coating to be carried out efficiently by a method such as printing or dispensing.

There are some metal powder sintering pastes that contain resins. However, from the perspectives that sintering requires high temperature and a resin component or the volatile portion thereof contaminates peripheral members, the better ones are those that are free from resin and contain a volatile organic solvent as a dispersion medium. However, since the volatile organic solvent that is often used for metal powder sintering pastes has a lower viscosity than that of an adhesive containing resin, fine irregularities of the substrate-side electrode surface to be joined often cause an organic solvent component to wet/spread. In this case, bleeding occurs in an unexpected portion in the substrate, which causes the organic solvent to leak onto the substrate bottom surface during the process or results in wire bonding defects due to contamination. On the other hand, in the metal powder sintering paste of the present embodiment, since it contains an anionic surfactant, even when it contains an organic solvent, the organic solvent can be prevented from bleeding.

The dispersion medium may be one type of organic solvent or a mixture of two or more types of organic solvents but is preferably a mixture of diol and ether. This is because a metal powder sintering paste containing such a dispersion medium used therein can join an element and a mounting member together at a lower temperature.

The boiling point of the dispersion medium is preferably not more than 300° C., more preferably 150° C. to 250° C. This is because when the boiling point of the dispersion medium is in the range of 150° C. to 250° C., the change in viscosity of the metal powder sintering paste at room temperature, which is caused by volatilization of the dispersion medium, can be prevented and thus a good workability is obtained. Furthermore, when the dispersion medium has a boiling point in that range, the dispersion medium can be completely volatilized by calcining.

Examples of diol include: aliphatic diols such as ethylene glycol, 1,3-propanediol, 1,4-butanediol, diethylene glycol, 1,5-pentanediol, 1,6-hexanediol, dipropylene glycol, triethylene glycol, tetraethylene glycol, 1,2-propanediol, 1,3-butanediol, 2,3-butanediol, neopentyl glycol(2,2-dimethylpropane-1,3-diol), 1,2-hexanediol, 2,5-hexanediol, 2-methyl-2,4-pentanediol, 3-methyl-1,3-pentanediol, and 2-ethyl-1,3-hexanediol; alkylene oxide adducts of 2,2-bis(4-hydroxy cyclohexyl)propane and 2,2,-bis(4-hydroxy cyclohexyl)propane; and alicyclic diols such as 1,4-cyclohexanediol and 1,4-cyclohexanedimethanol.

Examples of ether include dipropylene glycol methyl ether, tripropylene glycol methyl ether, propylene glycol n-propyl ether, dipropylene glycol n-propyl ether, propylene glycol n-butyl ether, dipropylene glycol n-butyl ether, propylene glycol phenyl ether, dipropylene glycol dimethyl ether, 1,3-dioxolane, ethylene glycol monobutyl ether, diethylene glycol monoethyl ether, diethylene glycol monobutyl ether, and ethylene glycol monoethyl ether.

When the dispersion medium is a mixture of diol and ether, the mass ratio of diol to ether is preferably diol:ether=7 to 9:2. This is because the metal powder sintering paste containing a mixture of such organic solvents used therein can join an element and a mounting member together at a lower temperature.

The content of the dispersion medium is not particularly limited since the required viscosity varies according to the method of applying the metal powder sintering paste. In order to control the porosity of the sintered joining layer that is obtained by calcining the metal powder sintering paste, the content of the dispersion medium is preferably up to 30% by mass with respect to the metal powder sintering paste.

[Resin]

The metal powder sintering paste is substantially free from resin. Examples of the resin include binders such as an epoxy resin and a phenolic resin as well as hardeners such as a polyamide resin.

<Method of Producing Metal Powder Sintering Paste>

The method of producing a metal powder sintering paste includes mixing an anionic surfactant with silver particles having an average particle diameter (a median diameter) of 0.3 μm to 5 μm, wherein substantially no resin is mixed together.

When the metal powder sintering paste further contains a dispersion medium, the method of producing a metal powder sintering paste includes mixing an anionic surfactant, silver particles having an average particle diameter of 0.3 μm to 5 μm, and a dispersion medium, wherein substantially no resin is mixed together.

In the method of producing a metal powder sintering paste, mixing can be carried out at room temperature and preferably a degassing process is included. Inclusion of the degassing process can prevent the joining strength from being reduced due to gas bubbles entering under the chip.

<Metal Powder Sintering Paste>

A preferred metal powder sintering paste is one with which a conductive material having an electric resistance of not more than 6 μΩ·cm can be obtained by calcining in an air oven at 170° C. for 60 minutes.

A preferred metal powder sintering paste is one with which the ratio of the wetting/spreading diameter of the dispersion medium to the original diameter of the paste is not higher than 1.4, when it is applied to a gold electrode with a surface roughness Ra of 0.04 μm, which then is left to stand for 20 minutes. This is because such a metal powder sintering paste improves the leakage onto the substrate bottom surface and the wire bonding defects caused by contamination and thereby makes it possible to produce reliable semiconductor devices.

The metal powder sintering paste is preferably one which is characterized in that the ratio of the wetting/spreading diameter of the dispersion medium to the original diameter of the paste is not higher than 3.0, when it is applied to a gold electrode with a surface roughness Ra of 0.48 μm, which then is left to stand for 20 minutes. This is because such a metal powder sintering paste improves the leakage onto the substrate bottom surface and the wire bonding defects caused by contamination and thereby makes it possible to produce reliable semiconductor devices.

<Method of Producing Conductive Material>

Furthermore, the present disclosure relates to a method of producing a conductive material, the method including a process of calcining a metal powder sintering paste of the present disclosure.

[Calcining Conditions]

The above-mentioned calcining may be carried out, for example, in an atmosphere such as a non-oxidizing atmosphere, an air atmosphere, a vacuum atmosphere, an oxygen atmosphere, a mixed gas atmosphere, or an airflow but preferably is carried out in an oxygen, ozone, or air atmosphere. This is because calcining in such an atmosphere accelerates the thermal diffusion of silver and thereby makes it possible to obtain a conductive material having a higher sintering strength.

In the present disclosure, the above-mentioned calcining is carried out preferably at a temperature in the range of 150° C. to 320° C. This is because when calcining is carried out in this temperature range, metal can be joined at a lower temperature than the melting point of the resin package on which a semiconductor element or the like is mounted. Furthermore, calcining is carried out more preferably at a temperature in the range of 160° C. to 260° C., further preferably at a temperature in the range of 170° C. to 195° C. This is because a lead frame, for which a conventional resin-containing adhesive is supposed to be used, contains a member that deteriorates at a temperature of 200° C. or higher.

Preferably, the above-mentioned calcining is carried out so that the ratio of the wetting/spreading diameter of the dispersion medium to the original diameter of the paste is not higher than 1.4, when the metal powder sintering paste is applied to a gold electrode with a surface roughness Ra of 0.04 μm, which then is left to stand for 20 minutes. Such a physical property can be obtained by adding the anionic surfactant described above.

Preferably, calcining is carried out so that the ratio of the wetting/spreading diameter of the dispersion medium to the original diameter of the paste is not higher than 3.0, when the metal powder sintering paste is applied to a gold electrode with a surface roughness Ra of 0.48 μm, which then is left to stand for 20 minutes. Such a physical property can be obtained by adding the anionic surfactant described above.

<Conductive Material>

The conductive material of the present disclosure can be obtained by calcining a metal powder sintering paste of the present disclosure. The porosity of the conductive material is preferably 5% by volume to 35% by volume, more preferably 5% by volume to 25% by volume, and further preferably 5% by volume to 20% by volume. The conductive material has the advantage of having a high joining strength.

Preferably, the conductive material of the present disclosure has an electrical resistance value of not higher than 50 μΩ·cm. This is because the lower the electric resistance thereof, the better the heat dissipation, and the less the power loss when it is used as an electrode. The electrical resistance value is more preferably not higher than 10 μΩ·cm and further preferably not higher than 6 μΩ·cm.

EXAMPLES

Hereinafter, with reference to examples, comparative examples, and reference examples, the metal powder sintering paste, the method of producing a metal powder sintering paste, the conductive material, and the method of producing a conductive material according to the present embodiments are described.

Example 1

Organic solvents, namely, 2-ethyl-1,3-hexanediol (0.58 g) and diethylene glycol monobutyl ether (0.14 g), and an anionic liquid surfactant (manufactured by Sanyo Chemical Industries, Ltd., BEAULIGHT LCA-H (product name), laureth-5 carboxylic acid, liquid at 25° C., 0.09 g) were stirred with a planetary centrifugal mixer (THINKY MIXER (AWATORIRENTARO) AR-250 (product name), manufactured by THINKY) for one cycle including one minute of stirring and 15 seconds of degassing. Thus, a solvent mixture was obtained.

Flaky silver particles (manufactured by Fukuda Metal Foil & POWDER Co., LTD., AgC-239 (product name), flake form, average particle diameter (median diameter): 2.7 μm, specific surface area: 0.7 m²/g, the content of the particles with a particle diameter of smaller than 0.3 μm: 1% by mass, and the content of the particles with a particle diameter of not larger than 0.5 μm: 3% by mass, 9.19 g) were weighed and then added to the solvent mixture described above. The mixture thus obtained was stirred with the planetary centrifugal mixer (THINKY MIXER (AWATORIRENTARO) AR-250 (product name), manufactured by THINKY) for one cycle including one minute of stirring and 15 seconds of degassing. Thus, a metal powder sintering paste was obtained (the content of the silver particles was 91.9% by mass).

Example 2

A metal powder sintering paste (the content of the silver particles was 91.9% by mass) was obtained in the same manner as in Example 1 except that an anionic liquid surfactant (manufactured by Sanyo Chemical Industries, Ltd., BEAULIGHT LCA-25NH (product name), laureth-4 carboxylic acid, liquid at 25° C., 0.09 g) was used as the liquid surfactant.

Example 3

A metal powder sintering paste (the content of the silver particles was 91.9% by mass) was obtained in the same manner as in Example 1 except that an anionic liquid surfactant (manufactured by Kao Corporation, Kaosera 8110 (product name), liquid at 25° C., 0.09 g) was used as the liquid surfactant.

Example 4

A metal powder sintering paste (the content of the silver particles was 91.9% by mass) was obtained in the same manner as in Example 1 except that an anionic liquid surfactant (manufactured by Sanyo Chemical Industries, Ltd., BEAULIGHT LCA-30D (product name), sodium laureth-4 carboxylate, liquid at 25° C., 0.09 g) was used as the liquid surfactant.

Example 5

A metal powder sintering paste (the content of the silver particles was 91.9% by mass) was obtained in the same manner as in Example 1 except that an anionic liquid surfactant (manufactured by Sanyo Chemical Industries, Ltd., BEAULIGHT LCA-25N (product name), sodium laureth-4 carboxylate, liquid at 25° C., 0.09 g) was used as the liquid surfactant.

Example 6

A metal powder sintering paste (the content of the silver particles was 91.9% by mass) was obtained in the same manner as in Example 1 except that an anionic liquid surfactant (manufactured by Sanyo Chemical Industries, Ltd., BEAULIGHT ECA (product name), sodium trideceth-4 carboxylate, liquid at 25° C., 0.09 g) was used as the liquid surfactant.

Example 7

A metal powder sintering paste (the content of the silver particles was 91.9% by mass) was obtained in the same manner as in Example 1 except that an anionic liquid surfactant (manufactured by NOF CORPORATION, OLEOYLSARCOSINE 221P (product name), N-oleoyl-N-methylglycine (oleoyl sarcosine), liquid at 25° C., 0.09 g) was used as the liquid surfactant.

Example 8

A metal powder sintering paste (the content of the silver particles was 91.9% by mass) was obtained in the same manner as in Example 1 except that an anionic liquid surfactant (manufactured by NOF CORPORATION, NONSOUL OK-1 (product name), potassium oleate (aqueous solution), liquid at 25° C., 0.09 g) was used as the liquid surfactant.

Example 9

A metal powder sintering paste (the content of the silver particles was 91.9% by mass) was obtained in the same manner as in Example 1 except that an anionic liquid surfactant (manufactured by NOF CORPORATION, SOFTILT®AS-L (product name), N-dodecanoyl-N-methyl-ß-alanine sodium salt, liquid at 25° C., 0.09 g) was used as the liquid surfactant.

Example 10

A metal powder sintering paste (the content of the silver particles was 91.9% by mass) was obtained in the same manner as in Example 1 except that an anionic liquid surfactant (manufactured by NOF CORPORATION, SOFTILT®AT-L (product name), N-dodecanoyl-N-methyl-6-alanine triethanolamine salt, liquid at 25° C., 0.09 g) was used as the liquid surfactant.

Example 11

A metal powder sintering paste (the content of the silver particles was 91.9% by mass) was obtained in the same manner as in Example 1 except that an anionic liquid surfactant (manufactured by NOF CORPORATION, FIRET®L (product name), N-lauroyl-N-methylglycine-sodium salt, liquid at 25° C., 0.09 g) was used as the liquid surfactant.

Example 12

A metal powder sintering paste (the content of the silver particles was 91.9% by mass) was obtained in the same manner as in Example 1 except that an anionic liquid surfactant (manufactured by NOF CORPORATION, NONSOUL OK-2 (product name), potassium oleate (aqueous solution), liquid at 25° C., 0.09 g) was used as the liquid surfactant.

Example 13

A metal powder sintering paste (the content of the silver particles was 91.9% by mass) was obtained in the same manner as in Example 1 except that an anionic liquid surfactant (manufactured by NOF CORPORATION, NONSOUL LK-30 (product name), potassium cocoate (aqueous solution), liquid at 25° C., 0.09 g) was used as the liquid surfactant.

Example 14

A metal powder sintering paste (the content of the silver particles was 91.9% by mass) was obtained in the same manner as in Example 1 except that an anionic liquid surfactant (manufactured by Sanyo Chemical Industries, Ltd., BEAULIGHT SHAA (product name), sodium lauryl glycol carboxylate, liquid at 25° C., 0.09 g) was used as the liquid surfactant.

Example 15

A metal powder sintering paste (the content of the silver particles was 91.9% by mass) was obtained in the same manner as in Example 1 except that an anionic liquid surfactant (manufactured by Kao Corporation, LATEMUL ASK (product name), dipotassium alkenyl succinate, liquid at 25° C., 0.09 g) was used as the liquid surfactant.

Example 16

A metal powder sintering paste (the content of the silver particles was 91.9% by mass) was obtained in the same manner as in Example 1 except that an anionic liquid surfactant (manufactured by DKS Co. Ltd., NEO-HITENOL S-70 (product name), disodium polyoxyethylene alkyl sulfosuccinate, liquid at 25° C., 0.09 g) was used as the liquid surfactant.

Example 17

A metal powder sintering paste (the content of the silver particles was 91.9% by mass) was obtained in the same manner as in Example 1 except that an anionic liquid surfactant (manufactured by Sanyo Chemical Industries, Ltd., BEAULIGHT ESS (product name), disodium C12-14 pareth-2 sulfosuccinate, liquid at 25° C., 0.09 g) was used as the liquid surfactant.

Example 18

A metal powder sintering paste (the content of the silver particles was 91.9% by mass) was obtained in the same manner as in Example 1 except that an anionic liquid surfactant (manufactured by Sanyo Chemical Industries, Ltd., BEAULIGHT A-5000 (product name), disodium lauramido peg-5 sulfosuccinate, liquid at 25° C., 0.09 g) was used as the liquid surfactant.

Example 19

A metal powder sintering paste (the content of the silver particles was 91.9% by mass) was obtained in the same manner as in Example 1 except that an anionic liquid surfactant (manufactured by NOF CORPORATION, PERSOFT®SK (product name), alkyl sulfate ester sodium salt, liquid at 25° C., 0.09 g) was used as the liquid surfactant.

Example 20

A metal powder sintering paste (the content of the silver particles was 91.9% by mass) was obtained in the same manner as in Example 1 except that an anionic liquid surfactant (manufactured by NOF CORPORATION, NEWREX®R-25L (product name), linear alkylbenzene-sulfonic acid sodium salt, liquid at 25° C., 0.09 g) was used as the liquid surfactant.

Example 21

A metal powder sintering paste (the content of the silver particles was 91.9% by mass) was obtained in the same manner as in Example 1 except that an anionic liquid surfactant (manufactured by NOF CORPORATION, SUNBASE (product name), α-sulfofatty acid methyl-ester sodium salt, liquid at 25° C., 0.09 g) was used as the liquid surfactant.

Example 22

A metal powder sintering paste (the content of the silver particles was 91.9% by mass) was obtained in the same manner as in Example 1 except that an anionic liquid surfactant (manufactured by NOF CORPORATION, NISSAN TRAX®K-40 (product name), polyoxyethylene lauryl ether sulfate-sodium salt, liquid at 25° C., 0.09 g) was used as the liquid surfactant.

Example 23

A metal powder sintering paste (the content of the silver particles was 91.9% by mass) was obtained in the same manner as in Example 1 except that an anionic liquid surfactant (manufactured by DKS Co. Ltd., NEOGEN AO-90 (product name), sodium α-olefin sulfonate, powder at 25° C., 0.09 g) was used as the liquid surfactant.

Example 24

A metal powder sintering paste (the content of the silver particles was 91.9% by mass) was obtained in the same manner as in Example 1 except that an anionic liquid surfactant (manufactured by Kao Corporation, LATEMUL PS (product name), sodium alkane sulfonate, liquid at 25° C., 0.09 g) was used as the liquid surfactant.

Example 25

A metal powder sintering paste (the content of the silver particles was 91.9% by mass) was obtained in the same manner as in Example 1 except that an anionic liquid surfactant (manufactured by DKS Co. Ltd., PLYSURF DBS (product name), sodium alkyl phosphate, liquid at 25° C., 0.09 g) was used as the liquid surfactant.

Comparative Example 1

A metal powder sintering paste (the content of the silver particles was 91.9% by mass) was obtained in the same manner as in Example 1 except that a cationic liquid surfactant (manufactured by Sanyo Chemical Industries, Ltd., LEBON TM-16 (product name), cetrimonium chloride, liquid at 25° C., 0.09 g) was used as the liquid surfactant.

Comparative Example 2

A metal powder sintering paste (the content of the silver particles was 91.9% by mass) was obtained in the same manner as in Example 1 except that a nonionic liquid surfactant (manufactured by Sanyo Chemical Industries, Ltd., EMULMIN NL-70 (product name), laureth-7, liquid at 25° C., 0.09 g) was used as the liquid surfactant.

Comparative Example 3

A metal powder sintering paste (the content of the silver particles was 91.9% by mass) was obtained in the same manner as in Example 1 except that a nonionic liquid surfactant (manufactured by Kao Corporation, Kaosera 8200 (product name), liquid at 25° C., 0.09 g) was used as the liquid surfactant.

Comparative Example 4

Organic solvents, namely, 2-ethyl-1,3-hexanediol (0.65 g) and diethylene glycol monobutyl ether (0.16 g) were mixed together and then stirred with the planetary centrifugal mixer (THINKY MIXER (AWATORIRENTARO) AR-250 (product name), manufactured by THINKY) for one cycle including one minute of stirring and 15 seconds of degassing. Thus, a solvent mixture was obtained.

Flaky silver particles (manufactured by Fukuda Metal Foil & POWDER Co., LTD., AgC-239 (product name), 9.19 g) were weighed and then added to the solvent mixture. The mixture thus obtained was stirred with the planetary centrifugal mixer (THINKY MIXER (AWATORIRENTARO) AR-250 (product name), manufactured by THINKY) for one cycle including one minute of stirring and 15 seconds of degassing. Thus, a metal powder sintering paste was obtained (the content of the silver particles was 91.9% by mass).

Tables 1 and 2 indicate the results of the measurements of the bleed ratio, electric resistance, and joining strength of Examples (Exs.) 1 to 25 and Comparative Examples (Comp. Exs.) 1 to 4. Since those with an electric resistance of 10 μΩ·cm or higher, which was obtained as the result of the measurement of the electric resistance, cannot be expected to have a high joining property, they were partly excluded from the evaluation of the joining strength. Furthermore, Table 3 indicates the results of the measurements of the electric resistance that was measured with various calcining temperatures with respect to Example 1 that showed a good result in Table 1 and Comparative Example 4 that contained no surfactant added thereto. The measurements of the bleed ratio, electric resistance, and joining strength were carried out as follows.

Each metal powder sintering paste obtained as described above was applied onto a substrate having a gold electrode with a surface roughness Ra of 0.04 μm on its surface by a stamping method to have a diameter of 170±50 μm, which then was left to stand for 20 minutes. Thereafter, the diameter including the dispersion medium that had bled was measured, and then the bleed ratio was calculated as the ratio of the diameter after bleeding to the original diameter of the paste.

Each metal powder sintering paste obtained as described above was applied to a glass substrate (thickness: 1 mm) by the screen printing method to have a thickness of 100 μm. Each glass substrate with a conductive material composition applied thereto was heated in an air atmosphere for 60 minutes at 170° C. in the case of Tables 1 and 2 and at specified temperatures indicated in Table 3. The electric resistance of each wiring (a conductive material) thus obtained was measured with MCP-T600 (product name) (manufactured by Mitsubishi Chemical Corporation) by a four-terminal method.

Each metal powder sintering paste obtained as described above was applied onto a substrate having a silver electrode on its surface by the stamping method, and a semiconductor element that has a silver electrode on its reverse side and a sapphire substrate with an outer dimension of 500×300 μm and a thickness of 150 μm was mounted thereon. The substrate having the semiconductor element mounted thereon with the metal powder sintering paste interposed therebetween was heated with an air oven at 170° C. for 60 minutes and thereafter, it was cooled. Then, shear force was applied in the direction of removing the semiconductor element from the substrate and the strength at which the semiconductor element was detached was measured as the joining strength.

Moreover, surfactants used in Examples 1 to 15 and Comparative Examples 1 to 3 were subjected to volatility evaluation by TG-DTA, and the residue (% by mass) at 350° C. was additionally recorded in Tables 1 and 2. The TG-DTA was carried out using TG/DTA6300 manufactured by SII NanoTechnology Inc. and an aluminum open sample pan under the conditions including a sample mass of 5 mg, an air flow rate of 200 mL/min, an initial temperature of 30° C., and a temperature rising rate of 2° C./min, and then the mass of the residue obtained when the temperature reached 350° C. was calculated as a ratio thereof to the initial mass.

TABLE 1 Surfactant Properties Paste Properties Residue Electric Die Shear at 350° C. Bleed Resistance Strength Polarity Type Principal Component Rational Formula (wt %) Ratio (μΩ · cm) (MPa) Ex. 1 Anionic Carboxylic acid type C₁₂H₂₅O(CH₂CH₂O)_(n)CH₂COOH (n = 4) 2.2 1.55 4.28 44 Ex. 2 Anionic Carboxylic acid type C₁₂H₂₅O(CH₂CH₂O)_(n)CH₂COOH (n = 2.5) 2.2 1.27 4.08 48 Ex. 3 Anionic Carboxylic acid type C₁₂H₂₅O(CH₂CH₂O)_(n)CH₂COOH (n = 4.5) 3.3 1.49 4.53 47 Ex. 4 Anionic Carboxylic acid type C₁₂H₂₅O(CH₂CH₂O)_(n)CH₂COONa (n ≠ 3) 5.5 1.45 5.43 22 Ex. 5 Anionic Carboxylic acid type C₁₂H₂₅O(CH₂CH₂O)_(n)CH₂COONa (n ≠ 3) 5.2 1.41 5.40 17 Ex. 6 Anionic Carboxylic acid type C₁₃H₂₇O(CH₂CH₂O)_(n)CH₂COONa (n ≠ 3) 21.6 1.43 7.07 9 Ex. 7 Anionic Carboxylic acid type C₁₇H₃₃C(O)N(CH₃)CH₂COOH 17.2 1.44 5.49 26 Ex. 8 Anionic Carboxylic acid type C₈H₁₇CH═CHC₇H₁₄COOK 7.5 1.48 5.66 31 Ex. 9 Anionic Carboxylic acid type [R—C(O)N(CH₃)C₂H₄COO]⁻Na⁺ 8.2 1.43 5.65 22 Ex. 10 Anionic Carboxylic acid type [R—C(O)N(CH₃)C₂H₄COO]⁻NH⁺(C₂H₄OH)₃ 5.3 1.38 4.23 35 Ex. 11 Anionic Carboxylic acid type [C₁₁H₂₃—C(O)N(CH₃)CH₂COO]⁻Na⁺ 8.9 1.37 5.42 22 Ex. 12 Anionic Carboxylic acid type CH₃(CH₂)₇CH═CH(CH₂)₇COOK 12.7 1.47 6.41 15 Ex. 13 Anionic Carboxylic acid type R—COOK 16.7 1.43 6.74 17 Ex. 14 Anionic Carboxylic acid type C₁₀H₂₁CH(OH)CH₂OCH₂COONa 12.0 1.46 5.82 14 Ex. 15 Anionic Carboxylic acid type R—CH(COOK)CH₂COOK 28.0 1.48 5.84 10

TABLE 2 Surfactant Properties Paste Properties Residue Electric Die Shear at 350° C. Bleed Resistance Strength Polarity Type Principal Component Rational Formula (wt %) Ratio (μΩ · cm) (MPa) Ex. 16 Anionic Carboxylic-sulfonic RO(CH₂CH₂O)_(n)COCH₂CH(SO₃Na)—COONa(n ≠ 7) — 1.42 4.72 18 acid type Ex. 17 Anionic Carboxylic-sulfonic RO(CH₂CH₂O)_(n)COCH(SO₃Na)—CH₂COONa(n ≠ 2) — 1.45 15.87 — acid type Ex. 18 Anionic Carboxylic-sulfonic C₁₁H₂₃CONHCH₂CH₂O(CH₂CH₂O)_(n)COCH — 1.47 5.19 13 acid type (SO₃Na)—CH—COONa(n ≠ 5) Ex. 19 Anionic Sulfonic acid type ROSO₃Na — 1.52 4.63 13 Ex. 20 Anionic Sulfonic acid type C_(n)H_(2n+1)(C₆H₄)SO₃Na(n = 10−16) — 1.49 6.05 15 Ex. 21 Anionic Sulfonic acid type R—CH(—SO₃Na)—COOCH₃ — 1.49 42.12 — Ex. 22 Anionic Sulfonic acid type C₁₂H₂₅O(CH₂CH₂O)_(n)SO₃Na — 1.53 26.75  1 Ex. 23 Anionic Sulfonic acid type R—CH═CH—CH₂—SO₃Na — 1.49 35.26 — Ex. 24 Anionic Sulfonic acid type R—SO₃Na — 1.36 15.03 — Ex. 25 Anionic Phosphorus acid type ROPO(—OR)ONa — 1.38 7.37  9 Comp. Cationic — [C₁₆H₃₃—N(CH₃)₃]⁺Cl⁻ 1.4 1.70 56.54  5 Ex. 1 Comp. Nonionic — C₁₂H₂₅O(CH₂CH₂O)_(n)H (n ≠ 7) 0.3 2.29 5.36 32 Ex. 2 Comp. Nonionic — Unsaturated carboxylic acid ester 43.0  2.20 45.57 10 Ex. 3 Comp. None — — — 1.72 4.53 14 Ex. 4

TABLE 3 Calcining Temperature 172° C. 177° C. 182° C. 185° C. 192° C. Comp. No surfactant 5.2 μΩ · cm 5.0 μΩ · cm 4.5 μΩ · cm 4.2 μΩ · cm 3.9 μΩ · cm Ex. 4 Ex. 1 BEAULIGHT LCA-H 4.5 μΩ · cm 4.3 μΩ · cm 3.8 μΩ · cm 3.8 μΩ · cm 3.9 μΩ · cm

As indicated in Tables 1 and 2, as compared to Comparative Example 4, in which no surfactant was added, in Examples 1 to 25, in each of which an anionic surfactant was added, the bleed ratio was decreased and thus an improvement was made. On the other hand, in Comparative Example 1, in which a cationic surfactant was added, no change was observed, and in Comparative Examples 2 and 3, in each of which a nonionic surfactant was added, the bleed ratio adversely increased and was worsened. Thus, it is clear that an anionic surfactant improves the bleed resistance.

On the other hand, with respect to sinterability, among Examples 1 to 15, in each of which the surfactant was an anionic carboxylic acid type, only Examples 6 and 15 that had poor volatility with the residue at 350° C. exceeding 20% by mass had reduced sinterability as compared to Comparative Example 4, in which no surfactant was added. Furthermore, high joining strength exceeding 40 MPa was obtained only in Examples 1 to 3 having good volatility with the residue at 350° C. being not more than 5% by mass.

On the other hand, in Comparative Example 1, in which a cationic surfactant was used, the residue at 350° C. was not higher than 5% by mass but the joining strength was low, specifically 5 MPa. Among those in which a nonionic surfactant was used, Comparative Example 3 with poor volatility had a low joining strength, but Comparative Example 2 with high volatility had a higher joining strength. However, as compared to Examples 1 to 3, in each of which an anionic surfactant was used, they had better volatility but the joining strength thereof did not reach 40 MPa.

The results described above indicate that the use of an anionic surfactant improves the bleed resistance, and the use of a surfactant with high volatility can maintain or improve the joining strength. Specifically, the residue at 350° C. is preferably not more than 20% by mass, most preferably not more than 5% by mass. In Example 1, in which an improvement was observed, it was confirmed that a low resistance was achieved at lower temperature as indicated in Table 3, and it is considered to be sufficient that the calcining temperature is around 180° C., which is equivalent to that of a resin adhesive material.

Example 26

Organic solvents, namely, 2-ethyl-1,3-hexanediol (0.38 g) and diethylene glycol monobutyl ether (0.09 g), and an anionic liquid surfactant (manufactured by Sanyo Chemical Industries, Ltd., BEAULIGHT LCA-H (product name), laureth-5 carboxylic acid, liquid at 25° C., 0.05 g) were stirred with the planetary centrifugal mixer (THINKY MIXER (AWATORIRENTARO) AR-250 (product name), manufactured by THINKY) for one cycle including one minute of stirring and 15 seconds of degassing. Thus, a solvent mixture was obtained.

Flaky silver particles (manufactured by Fukuda Metal Foil & POWDER Co., LTD., AgC-239 (product name), flake form, average particle diameter (median diameter): 2.5 μm, specific surface area: 0.7 m²/g, the content of the particles with a particle diameter of smaller than 0.3 μm: 1% by mass, and the content of the particles with a particle diameter of not larger than 0.5 μm: 3% by mass, 5.00 g) were weighed and then added to the solvent mixture described above. The mixture thus obtained was stirred with the planetary centrifugal mixer (THINKY MIXER (AWATORIRENTARO) AR-250 (product name), manufactured by THINKY) for one cycle including one minute of stirring and 15 seconds of degassing. After stirring, a mesh (330T mesh, wire diameter: 40 μm) was used to filtrate it. Thus, a metal powder sintering paste was obtained (the content of the silver particles was 90.6% by mass).

Example 27

A metal powder sintering paste (the content of the silver particles was 90.6% by mass) was obtained in the same manner as in Example 26 except that an anionic liquid surfactant (manufactured by Sanyo Chemical Industries, Ltd., BEAULIGHT LCA-25NH (product name), laureth-4 carboxylic acid, liquid at 25° C., 0.05 g) was used as the liquid surfactant.

Example 28

A metal powder sintering paste (the content of the silver particles was 90.6% by mass) was obtained in the same manner as in Example 26 except that an anionic liquid surfactant (manufactured by Kao Corporation, Kaosera 8110 (product name), liquid at 25° C., 0.05 g) was used as the liquid surfactant.

Comparative Example 5

A metal powder sintering paste (the content of the silver particles was 90.6% by mass) was obtained in the same manner as in Example 26 except that a nonionic liquid surfactant (manufactured by Kao Corporation, Kaosera 8200 (product name), liquid at 25° C., 0.05 g) was used as the liquid surfactant.

Comparative Example 6

Organic solvents, namely, 2-ethyl-1,3-hexanediol (3.01 g) and diethylene glycol monobutyl ether (0.75 g) were mixed together and then stirred with the planetary centrifugal mixer (THINKY MIXER (AWATORIRENTARO) AR-250 (product name), manufactured by THINKY) for one cycle including one minute of stirring and 15 seconds of degassing. Thus, a solvent mixture was obtained.

Flaky silver particles (manufactured by Fukuda Metal Foil & POWDER Co., LTD., AgC-239 (product name), 40.00 g) were weighed and then added to the solvent mixture. The mixture thus obtained was stirred with the planetary centrifugal mixer (THINKY MIXER (AWATORIRENTARO) AR-250 (product name), manufactured by THINKY) for one cycle including one minute of stirring and 15 seconds of degassing. After stirring, a mesh (330T mesh, wire diameter: 40 μm) was used to filtrate it. Thus, a metal powder sintering paste was obtained (the content of the silver particles was 91.4% by mass).

Table 4 indicates the results of the measurements of the contact angle of the raw material surfactant with respect to the gold electrode, the bleed ratio of the paste itself, and the joining strength of Examples 26 to 28 and Comparative Examples 5 and 6. The contact angle of the raw material surfactant with respect to the gold electrode, the bleed ratio, and the joining strength were measured as follows.

With respect to each surfactant itself used in Examples 26 to 28 and Comparative Example 5, the contact angle with respect to the gold electrode with a surface roughness Ra of 0.04 μm was measured using a contact angle meter CA-X150.

Each metal powder sintering paste obtained as described above was applied onto a substrate having a gold electrode on its surface by the stamping method, and a semiconductor element that has a silver electrode on its reverse side and a sapphire substrate with an outer dimension of 500×300 μm and a thickness of 150 μm was mounted thereon. The substrate having the semiconductor element mounted thereon with the metal powder sintering paste interposed therebetween was heated with an air oven at 175° C. for 90 minutes and thereafter, it was cooled. Then, shear force was applied in the direction of removing the semiconductor element from the substrate and the strength at which the semiconductor element was detached was measured as the joining strength.

Each metal powder sintering paste obtained as described above was applied onto two types of substrates, each of which has a gold electrode on its surface and is different in surface roughness, by the stamping method to have a diameter of 170±50 μm, which then was left to stand for 20 minutes. Thereafter, the diameter including the dispersion medium that had bled was measured, and then the bleed ratio was calculated as the ratio of the diameter after bleeding to the original diameter of the paste.

TABLE 4 Ex. 26 Ex. 27 Ex. 28 Comp. Ex. 5 Comp. Ex. 6 Surfactant Name BEAULIGHT BEAULIGHT Kaosera Kaosera No Physical LCA-H LCA-25NH 8110 8200 Surfactant Property Polarity Anion Anion Anion Nonion — Contact Angle 13.3-14.7 10.6-13.5 10.2-13.3 6.6-8.4 — (degree) with respect to Gold Electrode with Ra 0.04 μm Joining Strength (MPa) 36 27 21 9 38 Bleed Gold Electrode 1.4 1.2 1.4 2.2 1.5 Ratio with Ra 0.04 μm Gold Electrode 2.6 1.7 3.0 3.7 3.3 with Ra 0.48 μm

As indicated in Table 4, as compared to Comparative Example 6, in which no surfactant was used, in Examples 26 to 28, in each of which an anionic surfactant with a contact angle of larger than 10 degrees with respect to the gold electrode with RA 0.04 μm was used, the bleed ratio was reduced in both cases of the gold electrode with Ra 0.04 μm and the gold electrode with Ra 0.48 μm and thus, it was confirmed that bleeding tends not to occur. Furthermore, under all conditions, the higher the roughness Ra of the gold electrode, the higher the bleed ratio. In Comparative Example 5, in which a nonionic surfactant with a contact angle of smaller than 10 degrees with respect to the gold electrode with Ra 0.04 μm was used, the bleed ratio increased and thus worse results were obtained. These results showed that bleeding can be prevented by adding a surfactant with a larger contact angle with respect to the electrode.

Moreover, with respect to the joining strength, a sufficiently high joining strength exceeding 20 MPa was obtained in Examples 26 to 28 as compared to Comparative Example 6, in which no surfactant was added.

The metal powder sintering pastes of the present embodiments can be used, for example, for producing heat-resistant power wiring, component electrodes, die attaches, microbumps, flat panels, solar wiring, etc., for wafer bonding, and for producing electronic components that are produced using these in combination. Furthermore, the method of producing a conductive material of the present embodiment also can be used, for example, for producing a light-emitting device in which a light-emitting element such as LED or LD is used.

The invention may be embodied in other forms without departing from the spirit or essential characteristics thereof. The embodiments disclosed in this application are to be considered in all respects as illustrative and not limiting. The scope of the invention is indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are intended to be embraced therein. 

The invention claimed is:
 1. A metal powder sintering paste, the paste comprising, as a principal component, silver particles having a median diameter of 0.3 μm to 5 μm, and further comprising an anionic surfactant, and being substantially free from resin, wherein in the silver particles, a content of particles with a particle diameter of smaller than 0.3 μm is not more than 5% by mass, wherein a content of the surfactant is not more than 2% by mass with respect to the paste, and wherein the surfactant is a saturated carboxylic acid represented by Formula (I) below: R¹O(CH₂CH(R²)O)_(n1)CH₂COOR³  (I) wherein R¹ is a linear or branched alkyl group having at least 7 carbon atoms, R² is any one of —H, —CH₃, —CH₂CH₃, and —CH₂CH₂CH₃, R³ is —H or alkali metal, and n1 is in a range of 2 to
 5. 2. The metal powder sintering paste according to claim 1, wherein the surfactant is one in which when the temperature is increased with thermogravimetric-differential thermal analysis (TG-DTA) from room temperature to 350° C. at 2° C./min, a residue is reduced to not more than 20% by mass with respect to the initial mass.
 3. The metal powder sintering paste according to claim 1, wherein the surfactant is liquid at 25° C.
 4. The metal powder sintering paste according to claim 1, wherein the silver particles are in a flake form.
 5. The metal powder sintering paste according to claim 1, wherein in the silver particles, the content of particles with a particle diameter of smaller than 0.5 μm is not more than 15% by mass.
 6. The metal powder sintering paste according to claim 1, wherein the metal powder sintering paste further comprises an organic solvent as a dispersion medium.
 7. The metal powder sintering paste according to claim 6, wherein the organic solvent has a boiling point in a range of 150 to 250° C.
 8. The metal powder sintering paste according to claim 1, capable of being sintered to form a conductive material having an electric resistance of not higher than 6 μΩ·cm.
 9. The metal powder sintering paste according to claim 1, wherein a content of the silver particles is at least 70% by mass with respect to the paste. 